Core poppler annot (Annot.cc) has cropbox offsets included
in the coordinates of the relevant fields (rect and
quadrilaterals fields).

This commit makes poppler-glib API _not_ include cropbox
offsets when providing annot info to clients (by substracting
cropbox offsets from the read core Annot info) and in the same
way, assumes no cropbox offsets are included in the info
received from clients to create new annots (cropbox offsets will
be automatically added to the corresponding core poppler Annot).

As a result of this, existent clients (like Evince) now automatically
work right for annotations placed in pages that have a cropbox.

Poppler issue: poppler/poppler#129
Evince issue: https://gitlab.gnome.org/GNOME/evince/issues/1280